[TRLog] Switch box

[Tony and Celia Becker]

Hi All!

At 10:43 AM 11/4/97 -0800, Tree  N6TR <n6tr@teleport.com> wrote:

>I found some 2.2 or 2.7K resistors and pulled all of the signals
>coming from the computer up to 5 volts with them.  This totally
>fixed the problem and ran for 1485 QSOs during the SS with no
>problems.

It is important to realize that the generic PC parallel port pin is designed to drive a printer input impedance of typically >100 KOhms and <10 pF at data rates of a few hundred KHz.  We we must plan for a minimum of only 50 uA of base current, which multiplied by the beta of the transistor must be greater than the load current needed for the collector to pull the PTT line down to ground, typically <5 mA.  

We need a transistor with a guaranteed beta of 50 or better.  This is very doable, and is probably not the real problem.

Problems can be related to the Strobe signal, pin 1, where all the emitters are tied, and all the base and collector currents converge.  Current sink specs are typically better than sourcing, but fairly limited too.  Some else, (w2vjn perhaps) said he adds an extra transistor pair at the Strobe pin, just to make certain he can sink all the emitter current from the PTT and CW transistors, and now has no problems.  It seems to me that this would be a more robust design, and less work too.

Don't use those RS cheapo grab bag transistors, I have learned this lesson the hard way on Field Day where the tools and workplace were very primative; use real, marked, 2N3904 / 2N3906 or equivalents, and you should be OK without the pullup resistors.

73

AE0M, Tony Becker - becker@sprintmail.com - Silicon Valley, USA

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